Conventionally, a torque limiter to be provided interventionally between a motor and a driven component in a steel rolling mill is described in JP 63-30527 B. In this torque limiter, in which an inner circumferential surface of a cylinder member is externally fitted to an outer circumferential surface of a shaft member, pressure oil is fed to a hydraulic passage of the cylinder member, causing the inner circumferential surface of the cylinder member to be contracted in diameter by the pressure oil of the hydraulic passage so that the inner circumferential surface is pressed against the outer circumferential surface of the shaft member, by which the shaft member and the cylinder member are frictionally coupled to each other to allow torque transmission to be fulfilled. Then, the pressure oil within the hydraulic passage is sealed by a shear tube, while an engagement member to engage with an end portion of the shear tube is fixed to the shaft member.
When the shaft member or the cylinder member is burdened with a load of a specified value or higher so that the inner circumferential surface of the cylinder member slips against the outer circumferential surface of the shaft member with the result that the shaft member is changed in its circumferential position relative to the cylinder member, the engagement member cuts off an end portion of the shear valve so that the pressure oil in the hydraulic passage is discharged outside. As a result, the inner circumferential surface of the cylinder member is no longer pressed against the outer circumferential surface of the shaft member, so that the frictional coupling between the shaft member and the cylinder member is released to cut off the torque transmission. Also, lubricating oil is filled between the shaft member and the cylinder member in order to prevent the shaft member and the cylinder member from seizing while the inner circumferential surface of the cylinder member is sliding in contact with the outer circumferential surface of the shaft member with a load of a specified value or more applied to the shaft member or the cylinder member.
However, in the conventional torque limiter, in which the seizure-preventing lubricating oil is filled, the force with which the cylinder member clamps the shaft member is weakened, so that the cylinder member is rotated relative to the shaft member even under an ordinary torque load, giving rise to a problem that the release torque (a torque at which a torque limiter starts idling) is lower than a desired (design value) release torque. Also, since the lubricating state by the filled lubricating oil is unstable, there is another problem that the release torque is not constant but unstable.    Patent Document 1: JP 63-30527 B